Compliant micro device transfer head

ABSTRACT

A compliant micro device transfer head and head array are disclosed. In an embodiment a micro device transfer head includes a spring portion that is deflectable into a space between a base substrate and the spring portion.

RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 15/157,247, filed May 17, 2016, which is acontinuation of U.S. patent application Ser. No. 14/723,231 filed May27, 2015, now U.S. Pat. No. 9,370,864, which is a continuation of U.S.patent application Ser. No. 13/466,966 filed May 8, 2012, now U.S. Pat.No. 9,105,492, which is incorporated herein by reference.

BACKGROUND

Field

The present invention relates to micro devices. More particularly,embodiments of the present invention relate to a micro device transferhead and a method of transferring an array of micro devices to adifferent substrate.

Background Information

Integration and packaging issues are one of the main obstacles for thecommercialization of micro devices such as integration of radiofrequency (RF) microelectromechanical systems (MEMS) microswitches,light-emitting diode (LED) integration onto image display systems, andMEMS or quartz-based oscillators.

Traditional technologies for transferring of devices include transfer bywafer bonding from a transfer wafer to a receiving wafer. One suchimplementation is “direct printing” involving one bonding step of anarray of devices from a transfer wafer to a receiving wafer, followed byremoval of the transfer wafer. Another such implementation is “transferprinting” involving two bonding/debonding steps. In transfer printing atransfer wafer may pick up an array of devices from a donor wafer, andthen bond the array of devices to a receiving wafer, followed by removalof the transfer wafer.

Some printing process variations have been developed where a device canbe selectively bonded and debonded during the transfer process. Still,in both traditional and variations of the direct printing and transferprinting technologies, the transfer wafer must be debonded from a deviceafter bonding the device to the receiving wafer. In addition, the entiretransfer wafer with the array of devices is involved in the transferprocess.

SUMMARY OF THE INVENTION

A compliant micro device transfer head and method of transferring anarray of micro devices to a different substrate are disclosed. Forexample, the receiving substrate may be, but is not limited to, adisplay substrate, a lighting substrate, a substrate with functionaldevices such as transistors, or a substrate with metal redistributionlines.

In an embodiment, a micro device transfer head includes a base substrateand a spring member. The spring member includes a spring anchor coupledto the base substrate and a spring portion deflectable into a spacebetween the spring portion and the base substrate. The spring portionalso comprises an electrode. A dielectric layer covers a top surface ofthe electrode. The spring portion may further comprise a mesa structurethat protrudes away from the base substrate, where the mesa structurehas tapered sidewalls and the electrode is formed on a top surface ofthe mesa structure. The mesa structure can be separately or integrallyformed with the spring portion.

An electrode lead may extend from the electrode in order to make contactwith wiring in the base substrate and connect the micro device transferhead to the working electronics of an electrostatic gripper assembly.The electrode leads can run from the electrode on the top surface of themesa structure and along a sidewall of the mesa structure. The electrodelead can alternatively run underneath the mesa structure and connect toa via running through the mesa structure to the electrode. The springportion may additionally comprise a second electrode and electrode lead.

In an embodiment, the micro device transfer head comprises a sensor tomeasure an amount of deflection of the spring portion. The sensor may becoupled to the spring member or formed within the spring member. Thesensor may comprise two electrodes, one formed on the bottom surface ofthe spring member and a second formed directly beneath the firstelectrode within the space underlying the spring portion of the springmember. The sensor may measure strain or capacitance to determine theamount of deflection of the spring portion. The amount of deflectionmeasured by the sensor may indicate, for example, whether the transferhead has made contact with a micro device, or whether contaminationexists between the surfaces of the micro device and the transfer head.In an embodiment, the sensor is configured to measure a resonantfrequency of the spring portion in order to determine whether a microdevice has been picked up by the transfer head.

The space underlying the spring portion may be a cavity in the surfaceof the base substrate. Alternatively, the spring portion may be elevatedabove the base substrate by the spring anchor. The spring member may bea spring arm having a first end coupled to the base substrate or springanchor, and a second end suspended above the cavity, wherein the springanchor comprises the first end and the spring portion comprises thesecond end. A mesa structure may be formed on the second end of thespring arm. The spring member may comprise multiple spring arms.Alternatively, the spring portion may also completely cover the cavity.The mesa structure may be formed on the top surface of the springportion, over a center of the cavity.

In an embodiment, a method for selective transfer of micro devicesincludes bringing an array of compliant micro device transfer heads,each comprising a deflection sensor, into contact with an array of microdevices. The amount of deflection of each transfer head may then bemeasured, and each transfer head may be selectively activated based onthe amount of deflection detected by a sensor in the transfer head, suchthat only those transfer heads whose deflection indicates contact withthe surface of a micro device are activated in order to pick up thecorresponding micro device.

In an embodiment, a method for selective pick up of an array of microdevices includes an array of micro device transfer heads where eachtransfer head includes an electrode on a backside of the spring portionand a corresponding electrode at the bottom of a cavity, opposite thebackside electrode. One of the backside or opposing electrodes may becovered by a dielectric layer to prevent shorting. When a transfer headis depressed, a voltage may be applied across the two electrodes to lockthe transfer head in the depressed position. To enable selectivetransfer, the transfer heads in an array may first be depressed andlocked in the depressed position. The voltage may then be selectivelyremoved from a portion of the transfer heads, releasing the selectedtransfer heads from the depressed position so that they are poised topick up micro devices. The transfer head array may be positioned abovean array of micro devices on a carrier substrate, and brought intocontact so that only the selectively released transfer heads contact andpick up a corresponding portion of micro devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view and isometric illustration of abipolar cantilever micro device transfer head in accordance with anembodiment of the invention.

FIG. 2 is an isometric illustration of a monopolar cantilever microdevice transfer head in accordance with an embodiment of the invention.

FIG. 3 is an isometric illustration of a bipolar cantilever micro devicetransfer head in accordance with an embodiment of the invention.

FIG. 4 is an isometric illustration of a cantilever bipolar micro devicetransfer head including conductive vias in accordance with an embodimentof the invention.

FIGS. 5A-B are top-down illustrations of a bipolar cantilever microdevice transfer head in accordance with an embodiment of the invention.

FIGS. 6A-D are cross-sectional side view illustrations of sensorcomponents of a cantilever micro device transfer head in accordance withan embodiment of the invention.

FIG. 7 is an isometric illustration of a bipolar cantilever micro devicetransfer head array in accordance with an embodiment of the invention.

FIG. 8 is an isometric illustration of a bipolar cantilever micro devicetransfer head array including a conductive ground plane in accordancewith an embodiment of the invention.

FIG. 9 is a cross-sectional side view illustration of a bipolarcantilever micro device transfer head array including a conductiveground plane in accordance with an embodiment of the invention.

FIG. 10 is an isometric illustration of a bipolar micro device transferhead comprising multiple spring arms in accordance with an embodiment ofthe invention.

FIG. 11 is a cross-sectional side view illustration of a bipolarmembrane micro device transfer head in accordance with an embodiment ofthe invention.

FIG. 12 is an overhead isometric illustration of a membrane micro devicetransfer head in accordance with an embodiment of the invention.

FIGS. 13A-E are cross-sectional side view illustrations of bipolarmembrane micro device transfer heads according to an embodiment of theinvention.

FIGS. 14A-E are cross-sectional side view illustrations of a method forforming a bipolar membrane micro device transfer head according to anembodiment of the invention.

FIGS. 15A-K are cross-sectional side view illustrations of a method forforming a bipolar membrane micro device transfer head according to anembodiment of the invention.

FIGS. 16A-D are cross-sectional side view illustrations of an elevatedmicro device transfer heads in accordance with an embodiment of theinvention.

FIG. 17 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention.

FIG. 18 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention.

FIG. 19 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention.

FIG. 20 is a flow chart illustrating a method of picking up andtransferring a selected portion of an array of micro devices from acarrier substrate to at least one receiving substrate in accordance withan embodiment of the invention.

FIG. 21 is a flow chart illustrating a method of picking up andtransferring a portion of an array of micro devices from a carriersubstrate to at least one receiving substrate based on information fromone or more sensors in each of the micro device transfer heads inaccordance with an embodiment of the invention.

FIGS. 22A-B are cross-sectional side view illustrations of an array ofmicro device transfer heads in contact with an array of micro LEDdevices in accordance with an embodiment of the invention.

FIG. 23 is a cross-sectional side view illustration of an array of microdevice transfer heads picking up an array of micro LED devices inaccordance with an embodiment of the invention.

FIG. 24 is a cross-sectional side view illustration of an array of microdevice transfer heads picking up a portion of an array of micro LEDdevices in accordance with an embodiment of the invention.

FIG. 25 is a cross-sectional side view illustration of an array of microdevice transfer heads with an array of micro LED devices positioned overa receiving substrate in accordance with an embodiment of the invention.

FIG. 26 is a cross-sectional side view illustration of an array of microdevices released onto a receiving substrate in accordance with anembodiment of the invention.

FIG. 27 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with a smaller widththan the top surface of the micro p-n diode in accordance with anembodiment of the invention.

FIG. 28 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with a larger width thanthe top surface of the micro p-n diode in accordance with an embodimentof the invention.

FIG. 29 is a cross-sectional side view illustration of a variety ofmicro LED structures including contact openings with the same width asthe top surface of the micro p-n diode in accordance with an embodimentof the invention.

FIG. 30 is a cross-sectional side view illustration of an array of microdevice transfer heads illustrating varying degrees of deflection of thespring portion of a transfer head during a pick up operation.

FIG. 31 is a cross-sectional side view illustration of an array of microdevice transfer heads where a portion of the transfer heads have beenlocked in the depressed position.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention describe a compliant micro devicetransfer head and head array, and a method of transferring a microdevice and an array of micro devices from a carrier substrate to areceiving substrate. The receiving substrate may be, but is not limitedto, a display substrate, a lighting substrate, a substrate withfunctional devices such as transistors or integrated circuits (ICs), ora substrate with metal redistribution lines. In some embodiments, themicro devices and array of micro devices described herein may be a microLED device, such as the structures illustrated in FIGS. 26-28 and thosedescribed in related U.S. patent application Ser. No. 13/372,222, whichis incorporated herein by reference. While some embodiments of thepresent invention are described with specific regard to micro LEDs, itis to be appreciated that embodiments of the invention are not solimited and that certain embodiments may also be applicable to othermicro devices such as diodes, transistors, integrated circuits (ICs),and MEMS.

In various embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over”, “to”, “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

The terms “micro” device or “micro” LED structure as used herein mayrefer to the descriptive size of certain devices or structures inaccordance with embodiments of the invention. As used herein, the terms“micro” devices or structures are meant to refer to the scale of 1 to100 μm. However, it is to be appreciated that embodiments of the presentinvention are not necessarily so limited, and that certain aspects ofthe embodiments may be applicable to larger, and possibly smaller sizescales.

In one aspect, embodiments of the invention describe a manner for masstransfer of an array of pre-fabricated micro devices with an array ofcompliant transfer heads. For example, the pre-fabricated micro devicesmay have a specific functionality such as, but not limited to, an LEDfor light-emission, silicon IC for logic and memory, and galliumarsenide (GaAs) circuits for radio frequency (RF) communications. Insome embodiments, arrays of micro LED devices which are poised for pickup are described as having a 10 μm by 10 μm pitch, or 5 μm by 5 μmpitch. At these densities a 6 inch substrate, for example, canaccommodate approximately 165 million micro LED devices with a 10 μm by10 μm pitch, or approximately 660 million micro LED devices with a 5 μmby 5 μm pitch. A transfer tool including an array of compliant transferheads matching an integer multiple of the pitch of the correspondingarray of micro LED devices can be used to pick up and transfer the arrayof micro LED devices to a receiving substrate. In this manner, it ispossible to integrate and assemble micro LED devices intoheterogeneously integrated systems, including substrates of any sizeranging from micro displays to large area displays, and at high transferrates. For example, a 1 cm by 1 cm array of micro device transfer headscan pick up and transfer more than 100,000 micro devices, with largerarrays of micro device transfer heads being capable of transferring moremicro devices. Each compliant transfer head in the array of complianttransfer heads may also be independently controllable, which enablesselective pick up and release of the micro devices.

In one aspect, embodiments of the invention describe a compliant microdevice transfer head and a method of transfer in which an array of themicro device transfer heads enable improved contact with an array ofmicro devices as compared to an array of incompliant transfer heads. Anarray of compliant micro device transfer heads, wherein each transferhead includes a spring member, is lowered onto an array of micro devicesuntil the transfer heads make contact with the micro devices. The springmember components of the compliant transfer heads can compensate forvariations in height of the micro devices or for particulatecontamination on top of a micro device. For example, without the springmembers it is possible that an array of transfer heads would not makecontact with each and every micro device in the array. An irregularmicro device height or a particle on a top surface of a single microdevice could prevent the remainder of the transfer heads from makingcontact with the remainder of the micro devices in the array. As aresult, an air gap could be formed between those transfer heads andmicro devices. With such an air gap, it is possible that the targetapplied voltage would not create a sufficient electrostatic force toovercome the air gap, resulting in an incomplete pick-up process. Inaccordance with embodiments of the invention, the spring membersassociated with taller or contaminated micro devices may deflect morethan spring members associated with shorter micro devices on a singletransfer substrate. In this manner, the spring members can alsocompensate for variations in height of the micro devices, assisting eachcompliant transfer head to make contact with each micro device, andensure that each intended micro device is picked up.

In one aspect, the compliant micro device transfer head structureincludes a sensor to monitor an amount of deflection of the springmember when the transfer head is brought into contact with a microdevice. The sensor may be used for a variety of reasons. In oneapplication, the sensor can be used to determine if contact has beenmade with a respective micro device. In another application, the sensorcan be used to detect an irregularly shaped or contaminated microdevice. In this manner, it may be determined whether to proceed toattempt to pick up the irregular or contaminated micro device.Additionally, it may be determined whether to apply a cleaning operationto the transfer head array or micro device array prior to reattempting apick up operation. In another application, the sensor can be used todetect whether a micro device is attached to the transfer head, and hassuccessfully been picked up.

In another aspect, a method for selective transfer of micro devicesincludes bringing an array of compliant micro device transfer heads,each comprising a deflection sensor, into contact with an array of microdevices. The amount of deflection of each transfer head may be measuredby the deflection sensor to determine whether the transfer head hascontacted a micro device, to indicate the presence of contamination orirregularities on the surface of the micro device, or indicate theabsence of a micro device. As such, each transfer head may beselectively activated based on the amount of deflection detected by thedeflection sensor, so that only those transfer heads whose deflectionindicate contact with the surface of a micro device are activated topick up the corresponding micro device.

In another aspect, a method for selective transfer of micro devicesincludes depressing the compliant transfer heads in an array, lockingthe transfer heads in the depressed position, and then selectivelyreleasing a portion of the transfer heads from the depressed position sothat each released transfer head may contact and pick up a correspondingmicro device in an array of micro devices.

Referring now to FIG. 1, an isometric view of a compliant transfer head100 with a monopolar electrode and a corresponding cross-sectional sideview of a compliant transfer head array are illustrated in accordancewith an embodiment of the invention. FIG. 2 is a close-up isometric viewof the spring member 110 shown in FIG. 1. The spring member feature ofthe micro device transfer head disclosed herein may be executed using avariety of structures that enable deflection of the transfer head.Exemplary embodiments include a cantilever beam (see, e.g., FIG. 1),multiple spring arms (see, e.g., FIG. 10), a membrane (see, e.g., FIG.11), and elevated platforms (see, e.g., FIGS. 16A-D). Other structuresmay be possible to enable a compliant transfer head. Additional featuresof a particular embodiment of a transfer head may be determined by thestructure of the spring member, such as the addition of a mesastructure, the placement of electrode leads, and the type and locationof deflection sensors. Accordingly, though features—such as thematerials and characteristics of the base substrate, spring member,electrode(s), and dielectric layer—are described with reference to thecantilever spring member structure shown in FIGS. 1 and 2, it is to beunderstood that certain features are equally applicable to other springmember structure embodiments subsequently described.

Each transfer head may include a base substrate 102, a spring member 110comprising a spring anchor 120 coupled to the base substrate 102 and aspring portion 122 comprising electrode 116, and a dielectric layer 113covering the top surface of the electrode. The spring portion 122 isdeflectable into a space 112 between the spring portion 122 and the basesubstrate 102. The dielectric layer 113 is not shown in the isometricview illustrations in FIGS. 1-2 so that the underlying elements may beillustrated. Spring portion 122 may include a spring arm 124 and a mesa104 including a top surface 108 and tapered sidewalls 106.

Base substrate 102 may be formed from a variety of materials such assilicon, ceramics and polymers that are capable of providing structuralsupport. In an embodiment, base substrate 102 has a conductivity between10³ and 10¹⁸ ohm-cm. Base substrate 102 may additionally includeinterconnect 130 to connect the micro device transfer head 100 to theworking electronics of an electrostatic gripper assembly via electrodelead 114.

Referring again to FIG. 1, spring portion 122 of spring member 110 isdeflectable into a space 112 separating spring portion 122 from the basesubstrate 102. In an embodiment, one end of spring member 110 comprisesthe spring anchor 120, by which spring member 110 is coupled to basesubstrate 102, and the other end comprises the spring portion 122suspended above space 112. In an embodiment, spring portion 122comprises spring arm 124, mesa structure 104, electrode 116, andelectrode lead 114. Spring arm 124 is formed from a material having anelastic modulus that enables deflection of spring portion 122 into space112 over the working temperature range of the micro device transferprocess. In an embodiment, spring arm 124 is formed from the same ordifferent material as base substrate 102, for example, semiconductormaterials such as silicon or dielectric materials such as silicondioxide and silicon nitride. In an embodiment, spring arm 124 isintegrally formed from base substrate 102, such as, during the etchingof space 112. In another embodiment, spring arm 124 is formed from alayer of material deposited, grown, or bonded onto base substrate 102.

In an embodiment, the material and dimensions of spring arm 124 areselected to enable spring portion 122 to deflect approximately 0.5 μminto space 112 when the top surface of transfer head 100 is subjected toup to 10 atm of pressure at operating temperatures up to 350° C.Referring to FIG. 2, spring arm 124 has a thickness T, width W, andlength L, according to an embodiment of the invention. In an embodiment,spring arm 124 is formed from silicon and has a thickness T of up to 1μm. The thickness T of spring arm 124 may be greater or less than 1 μm,depending on the elastic modulus of the material from which it isformed. In an embodiment, the width W of spring arm 124 is sufficient toaccommodate additional spring portion and transfer head elements, suchas electrode 116 and mesa structure 104. In an embodiment, the width Wof spring arm 124 may correspond to the size of the micro device to bepicked up. For example, where a micro device is 3-5 μm wide, the widthof the spring arm may also be 3-5 μm, and where a micro device is 8-10μm wide, the width of the spring arm may also be 8-10 μm. The length Lof spring arm 124 is long enough to enable deflection of spring portion122 given the modulus of the material from which spring arm 124 isformed, but less than the pitch of the transfer heads 100 in thetransfer head array. In an embodiment, the length L of spring arm 124may be from 8 to 30 μm.

Spring portion 122 of the cantilever spring member 110 shown in FIGS. 1and 2 further includes mesa structure 104 protruding away from basesubstrate 102. Mesa structure 104 has tapered sidewalls 106 and topsurface 108. Mesa structure 104 may be formed using any suitableprocessing technique, and may be formed from the same or differentmaterial than spring arm 124. In one embodiment, mesa structure 104 isintegrally formed with spring arm 124, for example by using casting orlithographic patterning and etching techniques. In an embodiment,anisotropic etching techniques can be utilized to form tapered sidewalls106 for mesa structure 104. In another embodiment, mesa structure 104may be deposited or grown, and patterned on top of the base substrate102. In an embodiment, mesa structure 104 is a patterned oxide layer,such as silicon dioxide, formed on a silicon spring arm 124.

In one aspect, the mesa structures 104 generate a profile that protrudesaway from the base substrate so as to provide a localized contact pointto pick up a specific micro device during a pick up operation. In anembodiment, mesa structures 104 have a height of approximately 1 μm to 5μm, or more specifically approximately 2 μm. Specific dimensions of themesa structures 104 may depend upon the specific dimensions of the microdevices to be picked up, as well as the thickness of any layers formedover the mesa structures. In an embodiment, the height, width, andplanarity of the array of mesa structures 104 on the base substrate 102are uniform across the base substrate so that each micro device transferhead 100 is capable of making contact with each corresponding microdevice during the pick up operation. In an embodiment, the width acrossthe top surface 126 of each micro device transfer head is slightlylarger, approximately the same, or less than the width of the topsurface of the each micro device in the corresponding micro device arrayso that a transfer head does not inadvertently make contact with a microdevice adjacent to the intended corresponding micro device during thepick up operation. As described in further detail below, sinceadditional layers may be formed over the mesa structure 104 (e.g.passivation layer 111, electrode 116, and dielectric layer 113) thewidth of the mesa structure may account for the thickness of theoverlying layers so that the width across the top surface 126 of eachmicro device transfer head is slightly larger, approximately the same,or less than the width of the top surface of the each micro device inthe corresponding micro device array.

Still referring to FIGS. 1 and 2, mesa structure 104 has a top surface108, which may be planar, and sidewalls 106. In an embodiment, sidewalls106 may be tapered up to 10 degrees, for example. Tapering the sidewalls106 may be beneficial in forming the electrodes 116 and electrode leads114 as described further below. A passivation layer 111 may cover thebase substrate 102 and array of spring arms 124 and mesa structures 104.In an embodiment, the passivation layer may be 0.5 μm-2.0 μm thick oxidesuch as, but not limited to, silicon oxide (SiO₂), aluminum oxide(Al₂O₃) or tantalum oxide (Ta₂O₅).

Spring member 110 further comprises electrode 116 and electrode lead114, according to an embodiment. In an embodiment, electrode 116 isformed on the top surface 108 of mesa structure 104. In an exemplaryembodiment, the top surface 108 of the mesa structure 104 onto whichelectrode 116 is formed is approximately 7 μm×7 μm in order to achieve a8 μm×8 μm top surface of the transfer head 100. In accordance with anembodiment, electrode 116 covers the maximum amount of surface area ofthe top surface 108 of the mesa structure 104 as possible whileremaining within patterning tolerances. Minimizing the amount of freespace increases the capacitance and resultant grip pressure that can beachieved by the micro device transfer head. While a certain amount offree space is illustrated on the top surface 108 of the mesa structure104 in FIGS. 1 and 2, electrode 116 may cover the entire top surface108. The electrode 116 may also be slightly larger than the top surface108, and partially or fully extend down the sidewalls 106 of the mesastructure 104 to ensure complete coverage of the top surface 108. It isto be appreciated that the mesa array may have a variety of differentpitches, and that embodiments of the invention are not limited to theexemplary 7 μm×7 μm top surface of the mesa structure 104 in a 10 μmpitch.

Electrode lead 114 may run from electrode 116 over the top surface 108of mesa structure 104, down sidewall 106 of the mesa structure, alongthe top surface of spring arm 124, and over spring anchor 120. In anembodiment, electrode lead 114 connects to interconnect 130 in basesubstrate 102, which may run through the base substrate to a back sideof the base substrate.

A variety of conductive materials including metals, metal alloys,refractory metals, and refractory metal alloys may be employed to formelectrode 116 and electrode lead 114. In an embodiment, electrode 116has a thickness up to 5,000 Å (0.5 μm). In an embodiment, the electrode116 includes a high melting temperature metal such as platinum or arefractory metal or refractory metal alloy. For example, an electrodemay include platinum, titanium, vanadium, chromium, zirconium, niobium,molybdenum, ruthenium, rhodium, hafnium, tantalum, tungsten, rhenium,osmium, iridium and alloys thereof. Refractory metals and refractorymetal alloys generally exhibit higher resistance to heat and wear thanother metals. In an embodiment, electrodes 116 are each an approximately500 Å (0.05 μm) thick layer of titanium tungsten (TiW) refractory metalalloy.

In an embodiment, a dielectric layer 113 covers electrode 116. Thedielectric layer 113 may also cover other exposed layers on transferhead 100 and base substrate 102. In an embodiment, the dielectric layer113 has a suitable thickness and dielectric constant for achieving therequired grip pressure of the micro device transfer head 100, andsufficient dielectric strength to not break down at the operatingvoltage. The dielectric layer 113 may be a single layer or multiplelayers. In an embodiment, the dielectric layer is 0.5 μm-2.0 μm thick,though the thickness may be more or less depending upon the specifictopography of the transfer head 100 and underlying mesa structure 104.Suitable dielectric materials may include, but are not limited to,aluminum oxide (Al₂O₃) and tantalum oxide (Ta₂O₅). In accordance withembodiments of the invention, the dielectric layer 113 possesses adielectric strength greater than the applied electric field so as toavoid shorting of the transfer head during operation. The dielectriclayer 113 can be deposited by a variety of suitable techniques such aschemical vapor deposition (CVD), atomic layer deposition (ALD) andphysical vapor deposition (PVD) such as sputtering. The dielectric layer113 may additionally be annealed following deposition. In oneembodiment, the dielectric layer 113 possesses a dielectric strength ofat least 400 V/μm. Such a high dielectric strength can allow for the useof a thinner dielectric layer. Techniques such as ALD can be utilized todeposit uniform, conformal, dense, and/or pin-hole free dielectriclayers with good dielectric strength. Multiple layers can also beutilized to achieve such a pin-hole free dielectric layer. Multiplelayers of different dielectric materials may also be utilized to formdielectric layer 113. In an embodiment, the underlying electrode 116includes platinum or a refractory metal or refractory metal alloypossessing a melting temperature above the deposition temperature of thedielectric layer material(s) so as to not be a limiting factor inselecting the deposition temperature of the dielectric layer 113. In anembodiment, following the deposition of the dielectric layer 113, a thincoating (not illustrated) may be formed over the dielectric layer 113 toprovide a specific stiction coefficient, so as to add lateral frictionand keep the micro devices from being knocked off the transfer headduring the pick up operation. In such an embodiment, the additional thincoating replaces top surface 126 as the contacting surface, and thissurface retains the dimensional array requirements described herein.Furthermore, the additional coating can affect the dielectric propertiesof the micro device transfer head which may affect the operability ofthe micro device transfer head. In an embodiment, the additional coatingthickness can be minimal (e.g. below 10 nm) so as to have little to noappreciable effect on the grip pressure.

Spring portion 122 is deflectable into the space 112 between springportion 122 and base substrate 102. In an embodiment, space 112 is acavity in the surface of base substrate 102. In another embodiment,spring portion 122 is elevated above base substrate 102 to create space112. In an embodiment, space 112 extends underneath the spring arm 124of spring portion 122. Space 112 may also comprise an undercut portionbeneath the top surface of base substrate 102. The dimensions of space112 are selected to enable deflection of spring portion 122 into space112, as discussed above with respect to spring arm 124.

FIG. 3 is a close-up isometric view of a spring member 110 having abipolar electrode, according to an embodiment of the invention. In anembodiment, electrodes 116A and 116B cover mesa structure 104. Forpurposes of clarity, the overlying dielectric layer is not illustrated.In an embodiment, electrodes 116A and 116B are formed over a passivationlayer (not shown) that covers mesa structure 104. In an exemplaryembodiment, where the top surface 108 of the mesa structure 104 isapproximately 7 μm×7 μm corresponding to a mesa array with a 10 μmpitch, the electrodes may cover the maximum amount of the surface areaof the top surface 108 of the mesa structure 104 as possible while stillproviding separation between electrodes 116A, 116B. The minimum amountof separation distance may be balanced by considerations for maximizingsurface area, while avoiding overlapping electric fields from theelectrodes. For example, the electrodes 116A, 116B may be separated by0.5 μm or less, and the minimum separation distance may be limited bythe height of the electrodes. In an embodiment, the electrodes arelonger than the top surface 108 in one direction, and partially or fullyextend down the sidewalls 106 of the mesa structure 104 to ensuremaximum coverage of the top surface 108. It is to be appreciated thatthe mesa array may have a variety of different pitches, and thatembodiments of the invention are not limited to the exemplary 7 μm×7 μmtop surface of the mesa structure 104 in a 10 μm pitch.

Electrode leads 114A and 114B connect to electrodes 116A and 116B,respectively, on the top surface 108 of mesa structure 104. Electrodes114 may run down a single inclined sidewall 106 of mesa structure 104and along the top surface of spring arm 124 to spring anchor 120. Theincline of sidewall 106 aids in the deposition and etching of metal toform electrode leads 114. In an embodiment, electrode leads 114A and114B are each located in proximity to the edges of spring arm 124 so asto permit the formation of a spring arm sensor (not shown) between theelectrode leads, on the top surface of spring arm 124. Electrode leads114A, 114B may be formed of the same or different conductive material aselectrodes 116A, 116B

Referring now to FIG. 4, an isometric view is provided of a springmember 110 having a bipolar electrode with an alternative electrode leadconfiguration in accordance with an embodiment of the invention. In suchan embodiment the electrode leads 114A, 114B run underneath a portion ofthe mesa structure 104, and conductive vias 117A, 117B run through themesa structure 104 (and an optional passivation layer not illustrated)connecting the electrodes 116A, 116B to the respective electrode leads114A, 114B. In such an embodiment, conductive vias 117A, 117B may beformed prior to formation of mesa structure 104, and may be formed ofthe same or different conductive material as electrode leads 114A, 114Band electrodes 116A, 116B. While vias 117A, 117B are illustrated withregard to a bipolar electrode structure in FIG. 4, it is to beappreciated that the above described via or vias may also be integratedinto monopolar electrode structures.

Referring now to FIGS. 5A-B, top view illustrations of electrodes 116A,116B of a bipolar micro device transfer head are provided in accordancewith embodiments of the invention. Thus far, mesa structure 104 has beendescribed as a single mesa structure as shown in FIG. 5A. However,embodiments of the invention are not so limited. In the embodimentillustrated in FIG. 5B, each electrode 116 is formed on a separate mesastructure 104A, 104B separated by a trench 105. An optional passivationlayer (not illustrated) may cover both mesa structures 104A, 104B.

FIGS. 6A-D each illustrate an embodiment of a micro device transfer head100 incorporating one or more sensors. Sensors can serve a variety ofpurposes during operation of the transfer head. For example, where asensor is used to measure an amount of deflection of the transfer head,this information can be used to determine if (1) contact has been madewith a micro device to be picked up, (2) contamination is present on themicro device, or alternatively the micro device has been damaged ordeformed, or (3) whether a micro device has been picked up.

FIGS. 6A-B illustrate cross sectional side views of a transfer headcomprising a strain sensor 128A/128B, according to an embodiment of theinvention. In an embodiment, strain sensor 128A/128B is a strain gaugecapable of measuring the amount of deflection of spring portion 122 intospace 112. When a transfer head contacts the surface of a micro deviceduring a pick up operation, it may deflect some amount in response tothe contact pressure. By measuring the amount of deflection of a springportion 122 and comparing it to the amount of deflection known toindicate clean contact with a micro device surface, strain sensor128A/128B can indicate whether transfer head 100 has contacted the topsurface of a micro device in an array and as such is ready to execute apick up operation. Detection of too little deflection may indicate thata micro device is absent from that position in the array, whiledetection of too much deflection may indicate separation or incompletecontact between the surface of the micro device and the surface of thetransfer head due to either the presence of contamination particles oran otherwise damaged or deformed micro device. In both cases, a voltagemay not be applied to the transfer head so as not to attempt to pick upthe absent or damaged micro device. In the case where contamination isdetected, a cleaning operation may be applied to the transfer head,micro device, or their respective array prior to reattempting the pickup operation.

In another embodiment, strain sensor 128A/128B is capable of measuringthe resonant frequency of spring portion 122. A spring arm 124 bearingthe weight of transfer head elements such as mesa structure 104,electrodes (not shown) and a dielectric layer (not shown), will have anatural resonant frequency. Upon picking up a micro device on thesurface of the transfer head, the resonant frequency will change due tothe additional weight of the micro device. In an embodiment, strainsensor 128A/128B can detect a change in the resonant frequency of springportion 122, which indicates that a micro device has been successfullypicked up by the transfer head.

In an embodiment, sensors 128A/B can be formed directly on or in basesubstrate 102. In an embodiment, sensors 128A/B can be formed on or in aspring layer 132 formed over substrate 102. For example, spring layer132 is silicon, in which case a passivation layer (not shown) is formedbetween strain sensor 128A and the interface of spring anchor 120 andspring portion 122 in order to isolate the sensor. In anotherembodiment, spring layer 132 is an oxide or nitride layer. Referring toFIG. 6A, in an embodiment, strain sensor 128A is formed on spring layer132 over the interface of spring anchor 120 and spring portion 122. Whenspring portion 122 deflects into space 112, strain along spring arm 124is not uniform; spring arm 124 experiences the maximum amount of strainat the interface of spring portion 122 and spring anchor 120. In anembodiment, strain sensor 128A spans the interface of spring anchor 120and spring portion 122, so as to be subject to the maximum amount ofstress associated with the deflection of spring portion 122.

In an embodiment, strain sensor 128A comprises a piezoelectric material.A piezoelectric material accumulates charge in response to an appliedmechanical stress. The accumulation of charge along strained surfaces ofa piezoelectric sensor can generate a measurable voltage related to theamount of strain. As such, as spring portion 122 deflects into space112, the voltage between the upper and lower surface of the strainsensor increases as the strain at the interface of spring anchor 120 andspring portion 122 increases, enabling calculation of the amount ofdeflection of spring portion 122. Piezoelectric materials include, forexample, crystalline materials such as quartz and ceramic materials suchas lead zirconate titanate (PZT).

In another embodiment, strain sensor 128A comprises a piezoresistivematerial. The electrical resistivity of a piezoresistive materialchanges in response to an applied mechanical stress. As such, strainsensor 128A may be subject to an electrical current, so that when springportion 122 deflects into space 112, the electrical resistivity ofstrain sensor 128A increases as the strain at the interface of springanchor 120 and spring portion 122 increases, causing a measurableincrease in the voltage across the sensor. The amount of deflection canbe calculated from the changes in voltage. Piezoresistive materialsinclude, for example, polycrystalline silicon, amorphous silicon,monocrystalline silicon, or germanium.

In an embodiment, strain sensor 128B is formed within the surface ofbase substrate 102 at the interface of spring anchor 120 and springportion 122, as shown in FIG. 6B. In an embodiment, spring anchor 120and spring arm 124 of spring portion 122 are formed from silicon. In anembodiment, portion of the interface of spring anchor 120 and springportion 122 is doped to form a piezoresistive strain sensor 128B. Forexample, the silicon surface may be doped with boron for a p-typematerial or arsenic for an n-type material. The changing mobility ofcharge carriers when the doped sensor region is strained gives rise tothe piezoresistive effects.

In another aspect, strain sensor 128A, 128B is used to measure theresonant frequency of spring portion 122. In an embodiment, springportion 122 oscillates at a resonant frequency determined in part by theweight of the elements forming spring portion 122. The oscillationresults in a correspondingly oscillating amount of strain at theinterface of spring anchor 120 and spring portion 122. After a microdevice has been picked up by the transfer head, the additional weight ofthe micro device will change the resonant frequency of spring portion122, resulting in changes in the oscillating strain at the interface ofspring anchor 120 and spring portion 122 that can be measured by strainsensor 128. In this manner, strain sensor 128A, 128B may be used todetermine if a transfer head has successfully picked up a micro deviceduring a pickup operation.

Referring to FIG. 6C, opposing electrodes are formed on each of springportion 122 and bulk substrate 102, according to an embodiment of theinvention. In an embodiment, the bottom surface of spring arm 124 facingspace 112 comprises a backside electrode 134. In an embodiment, backsideelectrode 134 is positioned on the bottom surface of spring arm 124opposite the mesa structure 104 formed on the top surface. In anembodiment, opposing electrode 138 is formed on base substrate 102,directly opposite backside electrode 134 within space 112. In anembodiment, dielectric layer 136 covers opposing electrode 138. Inanother embodiment, dielectric layer 136 covers backside electrode 134.

In an embodiment, electrodes 134 and 138 function as a capacitivesensor. The capacitance between two parallel conductors increases as thedistance between the conductors decreases. In an embodiment, a voltageis applied across electrodes 134 and 138. As the spring portion 122 isdepressed within space 112 toward base substrate 102, the distancebetween electrodes 134 and 138 decreases, causing the capacitancebetween them to increase. In this manner, the amount of deflection ofspring portion 122 can be calculated from changes in the capacitancebetween electrodes 134 and 138 across dielectric layer 136 and space112. Dielectric 136 prevents shorting between the electrodes when springportion 122 is fully depressed within space 112. The opposing electrodes134, 138 may be formed from any suitable conductive material, such asthose discussed above with respect to electrodes 116.

In another application, electrodes 134 and 138 may be used to measurethe resonant frequency of spring portion 122. As discussed above, in anembodiment, spring portion 122 oscillates at a resonant frequencydetermined in part by the weight of elements forming spring portion 122.The oscillation may result in a correspondingly oscillating capacitancebetween electrodes 134 and 138. After a micro device has been picked upby the transfer head, the additional weight of the micro device willchange the resonant frequency of spring portion 122, resulting inchanges in the oscillating capacitance as measured by electrodes 134 and138. In this manner, electrodes 134 and 138 may be used to determine ifa transfer head has successfully picked up a micro device during apickup operation.

In yet another application of the structure illustrated in FIG. 6C, theelectrodes 134 and 138, together with dielectric 136, are capable oflocking spring portion 122 in a fully depressed position. In anembodiment, prior to the pickup operation, the array of transfer headsmay be depressed to the point that backside electrode 134 contacts thesurface of dielectric 136. A voltage may then be applied betweenopposing electrode 138 and backside electrode 134, across dielectric136, locking spring portion 122 in the depressed position. In thedepressed position, the transfer heads may be “deflected” so that thetopography is reduced and the transfer heads are not in position forpickup. The voltage across the dielectric may then be selectivelyremoved for select transfer heads, which allows the spring arm to bereleased and return to the undeflected, neutral position. This positionmay correspond to a “selected” position, which has a higher topographyand “selected” transfer heads are in position for pick up of the microdevice.

In an embodiment illustrated in FIG. 6D, transfer head 100 comprisesboth a strain sensor 128A and electrodes 134, 138. The strain sensor128A and the electrodes 134, 138 may have different functions. Forexample, strain sensor 128A may measure deflection while electrodes 134,138 measure the resonant frequency of spring portion 122, or vice versa.In another embodiment (not shown), a transfer head 100 comprises both astrain sensor 128B, formed within spring anchor 120 and spring portion122, and electrodes 134, 138.

Referring now to FIGS. 7-9, an embodiment of the invention isillustrated in which a conductive ground plane is formed over thedielectric layer and surrounding the array of transfer heads. FIG. 7 isan isometric view illustration of an array of compliant micro devicetransfer heads 100 with a bipolar electrode configuration as previouslydescribed with regard to FIG. 3. For purposes of clarity, the optionalunderlying passivation layer and overlying dielectric layer have notbeen illustrated. Referring now to FIGS. 8-9, a conductive ground plane140 is formed over the dielectric layer 113 and surrounding the array oftransfer heads 100. The presence of ground plane 140 may assist in theprevention of arcing between transfer heads 100, particularly during theapplication of high voltages. Ground plane 140 may be formed of aconductive material which may be the same as, or different as theconductive material used to form the electrodes, or vias. Ground plane140 may also be formed of a conductive material having a lower meltingtemperature than the conductive material used to form the electrodessince it is not necessary to deposit a dielectric layer of comparablequality (e.g. dielectric strength) to dielectric layer 113 after theformation of ground plane 140.

FIG. 10 is an isometric view of a spring member structure where thespring portion comprises multiple spring arms, according to anembodiment of the invention. In an embodiment, spring member 110comprises spring portion 122 and multiple spring anchors 120A-D. In anembodiment, spring portion 122 comprises mesa structure 104 formed onspring platform 144, four spring arms 124A-D, two electrodes 116A-Bforming a bipolar electrode, and two electrode leads 114A-B. Springplatform 144 provides a structural base for the formation of mesastructure 104 and additional elements of the transfer head (e.g.,electrodes and dielectric layer).

In an embodiment, multiple spring arms 124A-D enable top surface 108 andthe additional device components formed thereon to remain level whenspring portion 122 is deflected into underlying space 112. A level topsurface of the transfer head may improve contact with the top surface ofa micro device during a pickup operation. In an embodiment, each springarm 124 extends from a corner of spring platform 144 and runs parallelto the edge of spring platform 144 before attaching to the basesubstrate at a spring anchor 120. By running the length of one edge ofspring platform 144, spring arms 124A-D have sufficient length to enablea desired degree of deflection of spring portion 122 into underlyingspace 112. In an embodiment, spring arms 124 have a thickness T lessthan their width W to ensure that the spring portion 122 deflectsdownward into underlying space 112 in response to pressure applied totop surface 108, while experiencing minimal torsional/lateraldeformation. The specific dimensions of spring arms 124 depend on themodulus of the material from which they are formed. Spring arms 124 maybe formed from any of the materials discussed above with respect tospring arm 124 in FIGS. 1-2.

Additionally, mesa structure 104 may have the characteristics discussedabove with respect to a mesa structure formed on a cantilever structurespring portion. In an embodiment, mesa structure 104 is formedintegrally with spring platform 144. In another embodiment, mesastructure 104 is formed over spring platform 144.

Still referring to the embodiment illustrated in FIG. 10, electrodeleads 114A, 114B each run from a respective electrode 116A, 116B, down atilted sidewall 106 of mesa structure 104 and along a respective springarm 124A, 124B to spring anchors 120A, 120B. In an embodiment, electrodeleads 114 connect the micro device transfer head to the workingelectronics of an electrostatic gripper assembly via interconnects inthe base substrate. It is to be understood that other electrode andelectrode lead configurations may be used in conjunction with a springmember having multiple spring arms, such as, a monopolar electrode (FIG.2) and electrode lead vias (FIG. 4), as discussed above with respect toa spring member having a single-arm cantilever structure.

Referring now to FIG. 11, a side view illustration is provided of acompliant micro device transfer head 200 having a spring member with amembrane structure and a bipolar electrode, along with a correspondingtransfer head array, according to an embodiment of the invention. Asshown, the bipolar device transfer head 200 may include a base substrate202, a spring member comprising spring anchor 220 coupled to basesubstrate 202 and a spring portion 222 comprising electrodes 216A/216B,and a dielectric layer 213 covering the top surface of electrodes216A/216B. The spring portion 222 is deflectable into a space 212between the spring portion 222 and the base substrate 202. In anembodiment of the invention, spring portion 222 additionally comprisesspring layer 266, and mesa structure 204 having top surface 208 andtapered sidewalls 206.

Base substrate 202 may be formed from a variety of materials such assilicon, ceramics and polymers that are capable of providing structuralsupport, as described above with respect to base substrate 102. Basesubstrate 202 may additionally include interconnect 230 to connect themicro device transfer head 200 to the working electronics of anelectrostatic gripper assembly via electrode lead 214A or 214B.

A top-down view of spring member 210 having a membrane structure isillustrated in FIG. 12, according to an embodiment of the invention. Inan embodiment, the spring anchor 220 comprises the full perimeter ofspring portion 222, at the interface of spring portion 222 and basesubstrate 202. In an embodiment, spring portion 222 comprises a mesastructure 204 that is centrally positioned with respect to spring anchor220. Other elements of spring member 210 have been omitted from FIG. 12for clarity.

Referring back to FIG. 11, spring portion 222 comprises spring layer266, mesa structure 204, electrodes 216A, 216B, and electrode leads214A, 214B. Spring layer 266 is formed from a material having an elasticmodulus that enables deflection of spring portion 222 into space 212over the working temperature range of the micro device transfer process.In an embodiment, spring layer 266 is formed from the same or differentmaterial as base substrate 202, for example, semiconductor materialssuch as silicon or dielectric materials such as silicon dioxide andsilicon nitride. In an embodiment, spring layer 266 is integrally formedfrom base substrate 202, such as, during the etching of space 212. Inanother embodiment, spring layer 266 is formed from a layer of materialbonded onto the surface of base substrate 202. An optional passivationlayer (not shown) may be formed over spring layer 266 in order toisolate spring layer 266 from electrodes 216. In an embodiment, springlayer 266 is from 0.5 μm to 2 μm thick.

In an embodiment, electrodes 216A, 216B are formed over spring layer 266and over the top surface 208 of mesa structure 204. Electrode leads214A, 214B may run from electrodes 216A, 216B along the top surface 209of spring layer 266, and over spring anchor 220. In an embodiment,electrode leads 214A, 214B connect to interconnect 230 in base substrate202. The materials and dimensions of electrodes 216A, 216B and electrodeleads 214A, 214B may be the same as described above with respect toelectrodes 116 and electrode leads 114.

Dielectric layer 213 is formed over the surface. In an embodiment, thedielectric layer 213 has a suitable thickness and dielectric constantfor achieving the required grip pressure of the micro device transferhead, and sufficient dielectric strength to not break down at theoperating voltage. The dielectric layer 213 may be a single layer ormultiple layers, and may be the same or different material as theoptional passivation layer. Suitable dielectric materials may include,but are not limited to, aluminum oxide (Al₂O₃) and tantalum oxide(Ta₂O₅), as described above with respect to dielectric layer 113. In anembodiment, dielectric layer 213 is from 0.5 to 2 μm thick. In anembodiment, top surface 226 of dielectric layer 213 over the mesastructure 204 corresponds to the top surface of the compliant microdevice transfer head 200.

In an embodiment space 212 is a cavity in the surface of base substrate202. In an embodiment, spring portion 222 completely covers space 212.Spring portion 222 is deflectable into space 212. The depth of space 212is determined by the amount of deflection desired for spring portion222, while the width of space 212 is determined by the pitch of thetransfer head array, as discussed above with respect to space 112. Thewidth of space 212 is less than the pitch of the transfer heads, butgreater than the top surface 226 of each transfer head 200.

In an embodiment, transfer head 200 further comprises backside electrode234 and opposing electrode 238. In an embodiment, backside electrode 234is formed on the lower surface of spring portion 222, underneath mesastructure 204. In an embodiment, opposing electrode 238 is formed withinspace 212 opposite backside electrode 234. In an embodiment, dielectriclayer 236 covers opposing electrode 238. In another embodiment,dielectric layer covers backside electrode 234. Electrodes 234 and 238may be operated so as to sense deflection of spring portion 222, tomonitor the resonant frequency of spring portion 222, and/or to lockspring portion 222 in the deflected position, as described above withrespect to backside electrode 134, opposing electrode 138, anddielectric 136 in FIG. 6C.

FIGS. 13A-E illustrate cross-sectional views of additional embodimentsof spring members having a membrane structure. In FIG. 13A, the layersof material forming spring portion 222 of spring member 210 are shapedto form a mesa structure 204, according to an embodiment of theinvention. In FIG. 13B, spring portion 222 further comprises grooves215, according to an embodiment of the invention. Grooves 215 may reducethe pressure required to deflect spring portion 222 into space 212. InFIG. 13C, mesa structure 204 is formed over spring layer 266, accordingto an embodiment of the invention. In an embodiment, electrodes 216A,216B are formed over mesa structure 204, and dielectric layer 213 coverselectrodes 216A, 216B.

In FIG. 13D, the compliant transfer head comprises strain sensor 228A.In an embodiment, strain sensor 228A is formed on dielectric layer 213,over the interface of spring anchor 220 and spring portion 222. When thespring portion 222 deflects into space 212 during a pick up operation,spring portion 222 and dielectric layer 213 deflect, straining strainsensor 228A. As such, the degree of deflection of membrane spring member210 during a pick up operation can be measured. In addition, strainsensor 228A can be used to detect changes in the resonant frequency ofspring portion 222 that indicate a micro device has been picked up bythe transfer head. Strain sensor 228A may be formed from a piezoelectricor piezoresistive material, as described above with respect to strainsensor 128A in FIG. 6A.

In FIG. 13E, the compliant transfer head comprises strain sensor 228B.In an embodiment, strain sensor 228B is formed within a silicon springlayer 266, spanning the interface of spring anchor 220 and springportion 222. When spring portion 222 deflects into space 212 during apick up operation, the spring layer 266 portion of spring portion 222deflects, straining strain sensor 228B. As such, the degree ofdeflection of spring member 210 during a pick up operation can bemeasured. In addition, as discussed above with respect strain sensors128A, 128B, and 228A, strain sensor 228B can be used to detect changesin the resonant frequency of spring portion 222 that indicate a microdevice has been picked up by the transfer head. In an embodiment, strainsensor 228B is formed from a piezoresistive material, as described abovewith respect to strain sensor 128B in FIG. 6B. In an embodiment wherespring layer 266 is silicon, strain sensor 228B may be formed by dopinga portion of spring layer 266.

FIGS. 14A-E illustrate a method for forming a micro device transferhead, according to an embodiment of the invention. In an embodiment, abase substrate 1402 having active zones 1454 is provided, as shown inFIG. 14A. A dielectric layer 1456 comprising interconnects 1452 andburied electrode 1438 is formed over base substrate 1402. Interconnects1452 and electrode 1438 each connect to active zones 1454 in basesubstrate 1402. A metal bump 1450 is formed over each interconnect 1452.In an embodiment, base substrate 1402 has the characteristics discussedabove with respect to base substrate 202. In an embodiment, basesubstrate 1402 is silicon. Active zones 1454 may be n-type or p-typedoped. Interconnects 1452 and buried electrode 1438 may be any suitableconductive material, such as Al or Cu. Metal bumps 1450 may be anysuitable conductive material, such as Cu or Au. In an embodiment, metalbumps 1450 are 2 μm thick.

Next, a handle substrate 1460 having an oxide layer 1462 and a springlayer 1466 is provided, as shown in FIG. 14B. An SOI substrate may beused, wherein handle substrate 1460 is a silicon wafer, oxide layer 1462is silicon oxide, and spring layer 1466 is silicon. In an embodiment,oxide layer 1462 is approximately 2 μm thick. In an embodiment, springlayer 1466 is 0.5 μm to 1 μm thick. In an embodiment, the spring layer1466 is coupled to metal bumps 1450 via metal pads 1464. In anembodiment, metal pads 1464 are Au and up to 1 μm thick. In anembodiment, backside electrode 1434 is formed on the surface of springlayer 1466 between metal pads 1464. Backside electrode 1434 may be anysuitable conductive material, as discussed above with respect tobackside electrode 234 in FIG. 11.

Spring layer 1466 is bound to metal bumps 1450 via metal pads 1464,creating space 1412 between the surfaces of spring layer 1466 anddielectric layer 1456, according to an embodiment. In an embodiment, thesubsequently formed spring portion comprising spring layer 1466 will bedeflectable into space 1412. In an embodiment, space 1412 isapproximately 2 μm thick, corresponding to the 2 μm thickness of metalbumps 1450. In an embodiment, backside electrode 1434 is aligned overburied electrode 1438.

In an embodiment, handle substrate 1460 is then removed. Handlesubstrate 1460 may be removed by any appropriate method, such aschemical-mechanical polishing (CMP) or wet etch. In an embodiment, oxidelayer 1462 is patterned to form mesa structure 1404 on the surface ofspring layer 1466, as shown in FIG. 14C. Oxide layer 1462 may bepatterned by any appropriate method known in the art. In an embodiment,oxide layer 1462 is completely removed from the surfaces of spring layer1466 that are adjacent to mesa structure 1404. A passivation layer 1411may then be formed over the top surface of spring layer 1466 and mesastructure 1404, as shown in FIG. 14D. Passivation layer 1411electrically isolates metal electrodes 1416 from spring layer 1466 toprevent shorting. In an embodiment, passivation layer 1411 is 500 Åthick. Passivation layer 1411 may be any suitable insulating dielectricmaterial, such as Al₂O₃ or Ta₂O₅. In another embodiment, oxide layer1462 is etched to leave a thin portion of oxide material covering thesurface of spring layer 1466 between mesa structures 1404. In such acase, passivation layer 1411 may be omitted.

A layer of metal is then blanket deposited over the surface ofpassivation layer 1411 and patterned to form electrodes 1416A, 1416B andsignal lines 1417. Electrodes 1416 and signal lines 1417 may be formedfrom a conductive material such as those discussed above with respect toelectrodes 116A, 116B.

Dielectric layer 1413 is then blanket deposited over the surface, asshown in FIG. 14D. In an embodiment, the dielectric layer 1413 has asuitable thickness and dielectric constant for achieving the requiredgrip pressure of the micro device transfer head, and sufficientdielectric strength to not break down at the operating voltage. Thedielectric layer 1413 may be a single layer or multiple layers, and maybe the same or different material as passivation layer 1411. Suitabledielectric materials may include, but are not limited to, aluminum oxide(Al₂O₃) and tantalum oxide (Ta₂O₅), as described above with respect todielectric layer 113, 213. Dielectric layer 1413 is from 0.5 μm to 2 μmthick. In an embodiment, dielectric layer 1413 is a 0.5 μm thick layerof Al₂O₃. In an embodiment, dielectric layer 1413 is deposited by atomiclayer deposition (ALD).

FIG. 14D illustrates an embodiment of a micro device transfer headhaving a spring member with a membrane structure. The structure in FIG.14D may be further processed to form a transfer head having other springmember structures. For example, a portion of spring layer 1466,passivation layer 1411, and dielectric layer 1413 are patterned todefine a spring member with a multi spring-arm structure, as illustratedin cross-section by FIG. 14E. Spring layer 1466, passivation layer 1411,and dielectric layer 1413 may also be patterned to form a cantileverstructure, such as that described above with respect to FIGS. 1-2.

In the embodiments shown in each of FIGS. 14D and 14E, backsideelectrode 1434 and buried electrode 1438 form a capacitive sensor asdescribed above with respect to backside electrode 134 and opposingelectrode 138 in FIG. 6C, and backside electrode 234 and opposingelectrode 238 in FIG. 11. In another embodiment, backside electrode 1434and buried electrode 1438 enable the locking of the spring portion inthe deflected position, as described above with respect to backsideelectrode 134 and opposing electrode 138 in FIG. 6C, and backsideelectrode 234 and opposing electrode 238 in FIG. 11.

FIGS. 15A-K illustrate a cross sectional view of a method for forming amicro device transfer head having spring member with a membranestructure, according to an embodiment of the invention. Handle substrate1560 is provided, as shown in FIG. 15A. Handle substrate 1560 may beformed from a variety of materials such as silicon, ceramics andpolymers that are capable of providing structural support for subsequentformation of device layers. In an embodiment, handle substrate 1560 is asilicon wafer.

Next, mesa cavities 1561 are patterned in the surface of handlesubstrate 1560, according to an embodiment of the invention shown inFIG. 15B. Mesa cavities may be formed by any suitable process, such asphotolithography and etching. In an embodiment, patterning layer 1562 isformed over one surface of handle substrate 1560 for the patterning ofmesa cavities 1561. In an embodiment, patterning layer 1562 isphotoresist. In another embodiment, patterning layer 1562 is a hardmaskmaterial, such as silicon oxide or silicon nitride. In an embodiment,mesa cavities 1561 are spaced at intervals corresponding to the pitch ofthe micro device transfer head array, for example 5 μm to 10 μm,corresponding to an integer multiple of the array of micro devices to bepicked up. The dimensions of each mesa cavity 1561 are determined by thedesired dimensions of the top surface of the transfer head after theaddition of additional device components, such as the electrodes and thedielectric layer, as described above with respect to FIGS. 1-2 and 11.In an embodiment, mesa cavities 1561 are 7 μm×7 μm wide and 2 μm deep.After the etching of mesa cavities 1561, patterning layer 1562 mayremoved as shown in FIG. 15C.

A spring layer 1566 is then formed over the surface of patterned handlesubstrate 1560, according to an embodiment of the invention shown inFIG. 15D. Spring layer 1566 may be any material suitable to form thestructural basis of a membrane spring member in a micro device transferhead. The material and dimensions of spring layer 1566 are selected toenable the spring portion of the subsequently formed transfer head todeflect a desired amount under the operating conditions of the transferprocess, as discussed above with respect to spring layer 266. Springlayer 1566 may be an oxide or nitride layer. In an embodiment, springlayer 1566 is a grown thermal oxide on the surface of silicon handlesubstrate 1560. In another embodiment, spring layer 1566 is formed byplasma enhanced chemical vapor deposition (PECVD). In an embodiment,spring layer 1566 is 0.5 μm to 3 μm thick.

In an embodiment, grooves 1515 are etched into the surface of springlayer 1566. Grooves 1515 may reduce the pressure required to deflect thetransfer head spring portion that is subsequently formed comprisingspring layer 1566. In another embodiment, after the formation of springlayer 1566, the remaining volume of each mesa cavity 1561 is filled withmaterial, such as oxide or nitride (not shown), to be planar with thelower surfaces of spring layer 1566.

A base substrate 1502 having pits 1519 is then provided, as shown inFIG. 15E. Base substrate 1502 may be any of the materials describedabove with respect to base substrate 202. In an embodiment, pits 1519are each at least 20 μm wide and 2 μm deep. In an embodiment, the pitchof pits 1519 matches the pitch of mesa cavities 1561.

Handle substrate 1560 having spring layer 1566 thereon is then coupledto base substrate 1502, as shown in FIG. 15F. Spring layer 1566 and basesubstrate 1502 may be coupled by any suitable process, such as waferbonding. In an embodiment, the mesa cavities 1561 in spring layer 1566align with pits 1519 to enclose spaces 1512. Handle substrate 1560 isthen removed, leaving spring layer 1566 bonded to the surface of basesubstrate 1502 as shown in FIG. 15G. Handle substrate 1560 may beremoved by any suitable process or processes, such as CMP and wet etch.The removal of handle substrate 1560 leaves a membrane of spring layer1566 having a mesa structure 1504 covering each space 1512.

Metal layer 1563 is then formed over the surface of spring layer 1566,as shown in FIG. 15H. Metal layer 1563 may be formed from any suitablemetal or layer of metals that adheres well to the underlying springlayer 1566, as discussed above with respect to materials for electrodes116, 216. In an embodiment, metal layer 1563 is formed by sputterdeposition. Metal layer 1563 is up to 0.5 μm thick. In an embodiment,metal layer 1563 is a 500 Å thick layer of TiW.

Metal layer 1563 is then patterned to form electrodes, according to anembodiment of the invention. Metal layer 1563 may be patterned byforming mask 1565 on the surface of metal layer 1563, as shown in FIG.15I. Metal layer 1563 is then etched to form electrodes 1516A, 1516B andelectrode leads 1514A, 1514B, as shown in FIG. 15J.

Next, dielectric layer 1513 is formed over spring layer 1566 andelectrodes 1516.

Dielectric layer 1513 has the properties described above with respect todielectric layer 113, 213. In an embodiment, dielectric layer 1513 is a0.5 μm thick layer of Al₂O₃. In an embodiment, dielectric layer 1513 isdeposited by atomic layer deposition (ALD).

FIGS. 16A-D illustrate cross-sectional views of micro device transferhead structures wherein the spring portion is elevated above the surfaceof the base substrate. In an embodiment, spring portion 1622 comprisesspring layer 1624 to create a spring member 1610 with a cantileverstructure. Spring layer 1624 is elevated above the top surface of basesubstrate 1602 to create space 1612, as shown in FIG. 16A. Springportion 1622 is deflectable into space 1612. Spring portion 1622 furthercomprises one or more electrodes 1616. Dielectric layer 1613 coversspring member 1610.

In another embodiment, spring portion 1622 is elevated above the topsurface of base substrate 1602 by spring anchors 1620A and 1620B, asshown in FIG. 16B. Spring portion 1622 additionally comprises electrodes1616A and 1616B. Dielectric layer 1613 is formed over the top surface ofspring member 1610. Space 1612 is formed between spring portion 1622 andbase substrate 1602. Spring portion 1622 is deflectable into space 1612.

In another embodiment, spring portion 1622 further comprises mesastructure 1604 on spring layer 1624. An embodiment of a spring member1610 with a cantilever structure, wherein the spring portion comprises amesa structure 1604 is shown in FIG. 16C. An embodiment of a springmember 1610 with a table structure, wherein the spring portion comprisesa mesa structure 1604 is shown in FIG. 16D.

FIG. 17 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention. Atoperation 1710 a compliant transfer head is positioned over a microdevice connected to a carrier substrate. The compliant transfer head maycomprise a base substrate, a spring member including a spring anchorcoupled to the base substrate and a spring portion comprising anelectrode where the spring portion is deflectable into a space betweenthe spring portion and the base substrate, and a dielectric layercovering the top surface of the electrode as described in the aboveembodiments. The transfer head may have a monopolar or bipolar electrodeconfiguration and a cantilever or membrane spring member structure, aswell as any other structural variations as described in the aboveembodiments. The micro device is contacted with the compliant transferhead at operation 1720. In an embodiment, the micro device is contactedwith the dielectric layer of the transfer head. In an alternativeembodiment, the transfer head is positioned over the micro device with asuitable air gap separating them which does not significantly affect thegrip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). Atoperation 1730 a voltage is applied to the electrode to create a grippressure on the micro device, and the micro device is picked up with thetransfer head at operation 1740. The micro device is then released ontoa receiving substrate at operation 1750.

While operations 1710-1750 have been illustrated sequentially in FIG.17, it is to be appreciated that embodiments are not so limited and thatadditional operations may be performed and certain operations may beperformed in a different sequence. For example, in one embodiment, aftercontacting the micro device with the transfer head, the transfer head isrubbed across a top surface of the micro device in order to dislodge anyparticles which may be present on the contacting surface of either ofthe transfer head or micro device. In another embodiment, an operationis performed to create a phase change in the bonding layer connectingthe micro device to the carrier substrate prior to or while picking upthe micro device. If a portion of the bonding layer is picked up withthe micro device, additional operations can be performed to control thephase of the portion of the bonding layer during subsequent processing.

Operation 1730 of applying the voltage to the electrode to create a grippressure on the micro device can be performed in various orders. Forexample, the voltage can be applied prior to contacting the micro devicewith the transfer head, while contacting the micro device with thetransfer head, or after contacting the micro device with the transferhead. The voltage may also be applied prior to, while, or after creatingthe phase change in the bonding layer.

The micro device may be any of the micro LED device structuresillustrated in FIGS. 27-29, and those described in related U.S. patentapplication Ser. No. 13/372,222. For example, referencing FIG. 27, amicro LED device 300 may include a micro p-n diode 335, 350 and ametallization layer 320, with the metallization layer between the microp-n diode 335, 350 and a bonding layer 310 formed on a substrate 301. Inan embodiment, the micro p-n diode 335, 350 includes a top n-doped layer314, one or more quantum well layers 316, and a lower p-doped layer 318.The micro p-n diodes can be fabricated with straight sidewalls ortapered sidewalls. In certain embodiments, the micro p-n diodes 350possess outwardly tapered sidewalls 353 (from top to bottom). In certainembodiments, the micro p-n diodes 335 possess inwardly tapered sidewalls353 (from top to bottom). The metallization layer 320 may include one ormore layers. For example, the metallization layer 320 may include anelectrode layer and a barrier layer between the electrode layer and thebonding layer. The micro p-n diode and metallization layer may each havea top surface, a bottom surface and sidewalls. In an embodiment, thebottom surface 351 of the micro p-n diode 350 is wider than the topsurface 352 of the micro p-n diode, and the sidewalls 353 are taperedoutwardly from top to bottom. The top surface of the micro p-n diode 335may be wider than the bottom surface of the p-n diode, or approximatelythe same width. In an embodiment, the bottom surface 351 of the microp-n diode 350 is wider than the top surface 321 of the metallizationlayer 320. The bottom surface of the micro p-n diode may also be widerthan the top surface of the metallization layer, or approximately thesame width as the top surface of the metallization layer.

A conformal dielectric barrier layer 360 may optionally be formed overthe micro p-n diode 335, 350 and other exposed surfaces. The conformaldielectric barrier layer 360 may be thinner than the micro p-n diode335, 350 metallization layer 320 and optionally the bonding layer 310 sothat the conformal dielectric barrier layer 360 forms an outline of thetopography it is formed on. In an embodiment, the micro p-n diode 335,350 is several microns thick, such as 3 μm, the metallization layer 320is 0.1 μm-2 μm thick, and the bonding layer 310 is 0.1 μm-2 μm thick. Inan embodiment, the conformal dielectric barrier layer 360 isapproximately 50-600 angstroms thick aluminum oxide (Al₂O₃). Conformaldielectric barrier layer 360 may be deposited by a variety of suitabletechniques such as, but not limited to, atomic layer deposition (ALD).The conformal dielectric barrier layer 360 may protect against chargearcing between adjacent micro p-n diodes during the pick up process, andthereby protect against adjacent micro p-n diodes from sticking togetherduring the pick up process. The conformal dielectric barrier layer 360may also protect the sidewalls 353, quantum well layer 316 and bottomsurface 351, of the micro p-n diodes from contamination which couldaffect the integrity of the micro p-n diodes. For example, the conformaldielectric barrier layer 360 can function as a physical barrier towicking of the bonding layer material 310 up the sidewalls and quantumlayer 316 of the micro p-n diodes 350. The conformal dielectric barrierlayer 360 may also insulate the micro p-n diodes 350 once placed on areceiving substrate. In an embodiment, the conformal dielectric barrierlayer 360 span sidewalls 353 of the micro p-n diode, and may cover aquantum well layer 316 in the micro p-n diode. The conformal dielectricbarrier layer may also partially span the bottom surface 351 of themicro p-n diode, as well as span sidewalls of the metallization layer320. In some embodiments, the conformal dielectric barrier layer alsospans sidewalls of a patterned bonding layer 310. A contact opening 362may be formed in the conformal dielectric barrier layer 360 exposing thetop surface 352 of the micro p-n diode.

Referring to FIG. 27, the contact opening 362 may have a smaller widththan the top surface 352 of the micro p-n diode and the conformaldielectric barrier layer 360 forms a lip around the edges of the topsurface 352 of the micro p-n diode. Referring to FIG. 28, the contactopening 362 may have a slightly larger width than the top surface of themicro p-n diode. In such an embodiment, the contact opening 362 exposesthe top surface 352 of the micro p-n diode and an upper portion of thesidewalls 353 of the micro p-n diode, while the conformal dielectricbarrier layer 360 covers and insulates the quantum well layer(s) 316.Referring to FIG. 29, the conformal dielectric layer 360 may haveapproximately the same width as the top surface of the micro p-n diode.The conformal dielectric layer 360 may also span along a bottom surface351 of the micro p-n diodes illustrated in FIGS. 27-29.

In an embodiment, conformal dielectric barrier layer 360 is formed ofthe same material as dielectric layer 113, 213 of the compliant transferhead. Depending upon the particular micro LED device structure, theconformal dielectric barrier layer 360 may also span sidewalls of thebonding layer 310, as well as the carrier substrate and posts, ifpresent. Bonding layer 310 may be formed from a material which canmaintain the micro LED device 300 on the carrier substrate 301 duringcertain processing and handling operations, and upon undergoing a phasechange provide a medium on which the micro LED device 300 can beretained yet also be readily releasable from during a pick up operation.For example, the bonding layer may be remeltable or reflowable such thatthe bonding layer undergoes a phase change from solid to liquid stateprior to or during the pick up operation. In the liquid state thebonding layer may retain the micro LED device in place on the carriersubstrate while also providing a medium from which the micro LED device300 is readily releasable. In an embodiment, the bonding layer 310 has aliquidus temperature or melting temperature below approximately 350° C.,or more specifically below approximately 200° C. At such temperaturesthe bonding layer may undergo a phase change without substantiallyaffecting the other components of the micro LED device. For example, thebonding layer may be formed of a metal or metal alloy, or athermoplastic polymer which is removable. For example, the bonding layermay include indium, tin or a thermoplastic polymer such as polyethyleneor polypropylene. In an embodiment, the bonding layer may be conductive.For example, where the bonding layer undergoes a phase change from solidto liquid in response to a change in temperature a portion of thebonding layer may remain on the micro LED device during the pick upoperation. In such an embodiment, it may be beneficial that the bondinglayer is formed of a conductive material so that it does not adverselyaffect the micro LED device when it is subsequently transferred to areceiving substrate. In this case, the portion of conductive bondinglayer remaining on the micro LED device during the transfer may aid inbonding the micro LED device to a conductive pad on a receivingsubstrate. In a specific embodiment, the bonding layer may be formed ofindium, which has a melting temperature of 156.7° C. The bonding layermay be laterally continuous across the substrate 301, or may also beformed in laterally separate locations. For example, a laterallyseparate location of the bonding layer may have a width which is lessthan or approximately the same width as the bottom surface of the microp-n diode or metallization layer. In some embodiments, the micro p-ndiodes may optionally be formed on posts 302 on the substrate.

Solders may be suitable materials for bonding layer 310 since many aregenerally ductile materials in their solid state and exhibit favorablewetting with semiconductor and metal surfaces. A typical alloy melts nota single temperature, but over a temperature range. Thus, solder alloysare often characterized by a liquidus temperature corresponding to thelowest temperature at which the alloy remains liquid, and a solidustemperature corresponding to the highest temperature at which the alloyremains solid. An exemplary list of low melting solder materials whichmay be utilized with embodiments of the invention are provided in Table1.

TABLE 1 Chemical composition Liquidus Solidus (weight %) Temperature (°C.) Temperature (° C.) 100In 156.7 156.7 66.3In33.7Bi 72 7251In32.5Bi16.5Sn 60 60 57Bi26In17Sn 79 79 54.02Bi29.68In16.3Sn 81 8167Bi33In 109 109 90In10Sn 151 143 48In52Sn 118 118 50In50Sn 125 11852Sn48In 131 118 58Sn42In 145 118 97In3Ag 143 143 94.5In5.5Ag 200 —99.5In0.5Au 200 — 95In5Bi 150 125 99.3In0.7Ga 150 150 99.4In0.6Ga 152152 99.6In0.4Ga 153 153 99.5In0.5Ga 154 154 58Bi42Sn 138 138 60Sn40Bi170 138 100Sn 232 232 95Sn5Sb 240 235 100Ga 30 30 99In1Cu 200 — 98In2Cu182 — 96In4Cu 253 — 74In26Cd 123 123 70In30Pb 175 165 60In40Pb 181 17350In50Pb 210 184 40In60Pb 231 197 55.5Bi44.5Pb 124 124 58Bi42Pb 126 12445.5Bi54.5Pb 160 122 60Bi40Cd 144 144 67.8Sn32.2Cd 177 177 45Sn55Pb 227183 63Sn37Pb 183 183 62Sn38Pb 183 183 65Sn35Pb 184 183 70Sn30Pb 186 18360Sn40Pb 191 183 75Sn25Pb 192 183 80Sn20Pb 199 183 85Sn15Pb 205 18390Sn10Pb 213 183 91Sn9Zn 199 199 90Sn10Au 217 217 99Sn1Cu 227 22799.3Sn0.7Cu 227 227

An exemplary list thermoplastic polymers which may be utilized withembodiments of the invention are provided in Table 2.

TABLE 2 Polymer Melting Temperature (° C.) Acrylic (PMMA) 130-140Polyoxymethylene (POM or Acetal) 166 Polybutylene terephthalate (PBT)160 Polycaprolactone (PCL)  62 Polyethylene terephthalate (PET) 260Polycarbonate (PC) 267 Polyester 260 Polyethylene (PE) 105-130Polyetheretherketone (PEEK) 343 Polylactic acid (PLA) 50-80Polypropylene (PP) 160 Polystyrene (PS) 240 Polyvinylidene chloride(PVDC) 185

FIG. 18 is a flow chart illustrating a method of picking up andtransferring a micro device from a carrier substrate to a receivingsubstrate in accordance with an embodiment of the invention. Atoperation 1810 a compliant transfer head is positioned over a microdevice connected to a carrier substrate with a bonding layer. Thecompliant transfer head may be any transfer head described herein. Themicro device may be any of the micro LED device structures illustratedin FIGS. 27-29 and those described in related U.S. ProvisionalApplication No. 61/561,706 and U.S. Provisional Application No.61/594,919. The micro device is then contacted with the transfer head atoperation 1820. In an embodiment, the micro device is contacted with thedielectric layer 113, 213 of the transfer head. In an alternativeembodiment, the transfer head is positioned over the micro device with asuitable air gap separating them which does not significantly affect thegrip pressure, for example, 1 nm (0.001 μm) or 10 nm (0.01 μm). Atoperation 1825 an operation is performed to create a phase change in thebonding layer 310 from solid to liquid state. For example, the operationmay include heating an In bonding layer at or above the meltingtemperature of 156.7° C. In another embodiment, operation 1825 can beperformed prior to operation 1820. At operation 1830 the micro device ispicked up with the compliant transfer head. For example, a voltage canbe applied to an electrode to create a grip pressure on the microdevice. A substantial portion of the bonding layer 310 may also bepicked up with the transfer head at operation 1840. For example,approximately half of the bonding layer 310 may be picked up with themicro device. In an alternative embodiment, none of the bonding layer310 is picked up with the transfer head. The micro device, andoptionally a portion of the bonding layer 310, is placed in contact witha receiving substrate. The micro device is then released onto thereceiving substrate at operation 1850. In accordance with an embodimentof the invention, a variety of operations can be performed to controlthe phase of the portion of the bonding layer when picking up,transferring, contacting the receiving substrate, and releasing themicro device and portion of the bonding layer 310 on the receivingsubstrate. For example, the portion of the bonding layer which is pickedup with the micro device can be maintained in the liquid state duringcontacting the receiving substrate and during the release operation1850. In another embodiment, the portion of the bonding layer can beallowed to cool to a solid phase after being picked up. For example, theportion of the bonding layer can be in a solid phase during contactingthe receiving substrate, and again melted to the liquid state prior toor during the release operation 1850. A variety of temperature andmaterial phase cycles can be performed in accordance with embodiments ofthe invention.

FIG. 19 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention. At operation 1910 an array of compliant transfer heads ispositioned over an array of micro devices. The compliant transfer headsmay be any transfer head described herein. At operation 1920 the arrayof micro devices are contacted with the array of transfer heads. In analternative embodiment, the array of transfer heads is positioned overthe array of micro devices with a suitable air gap separating them whichdoes not significantly affect the grip pressure, for example, 1 nm(0.001 μm) or 10 nm (0.01 μm). FIG. 22A is a side view illustration ofan array of micro device transfer heads 200 in contact with an array ofmicro LED devices 300 in accordance with an embodiment of the invention.As illustrated in FIG. 22A, the pitch (P) of the array of transfer heads200 matches the pitch of the micro LED devices 300, with the pitch (P)of the array of transfer heads being the sum of the spacing (S) betweentransfer heads and width (W) of a transfer head.

In one embodiment, the array of micro LED devices 300 have a pitch of 10μm, with each micro LED device having a spacing of 2 μm and a maximumwidth of 8 μm. In an exemplary embodiment, assuming a micro p-n diode350 with straight sidewalls the top surface of the each micro LED device300 has a width of approximately 8 μm. In such an exemplary embodiment,the width of the top surface 226 of a corresponding transfer head 200 isapproximately 8 μm or smaller so as to avoid making inadvertent contactwith an adjacent micro LED device. In another embodiment, the array ofmicro LED devices 300 may have a pitch of 5 μm, with each micro LEDdevice having a spacing of 2 μm and a maximum width of 3 μm. In anexemplary embodiment, the top surface of the each micro LED device 300has a width of approximately 3 μm. In such an exemplary embodiment, thewidth of the top surface 226 of a corresponding transfer head 200 isapproximately 3 μm or smaller so as to avoid making inadvertent contactwith an adjacent micro LED device 300. However, embodiments of theinvention are not limited to these specific dimensions, and may be anysuitable dimension.

FIG. 22B is a side view illustration of an array of micro devicetransfer heads in contact with an array of micro LED devices 300 inaccordance with an embodiment of the invention. In the embodimentillustrated in FIG. 22B, the pitch (P) of the transfer heads is aninteger multiple of the pitch of the array of micro devices. In theparticular embodiment illustrated, the pitch (P) of the transfer headsis 3 times the pitch of the array of micro LED devices. In such anembodiment, having a larger transfer head pitch may protect againstarcing between transfer heads.

Referring again to FIG. 19, at operation 1930 a voltage is selectivelyapplied to a portion of the array of transfer heads 200. Thus, eachtransfer head 200 may be independently operated. At operation 1940 acorresponding portion of the array of micro devices is picked up withthe portion of the array of transfer heads to which the voltage wasselectively applied. In one embodiment, selectively applying a voltageto a portion of the array of transfer heads means applying a voltage toevery transfer head in the array of transfer heads. FIG. 23 is a sideview illustration of every transfer head in an array of micro devicetransfer heads picking up an array of micro LED devices 300 inaccordance with an embodiment of the invention. In another embodiment,selectively applying a voltage to a portion of the array of transferheads means applying a voltage to less than every transfer head (e.g. asubset of transfer heads) in the array of transfer heads. FIG. 24 is aside view illustration of a subset of the array of micro device transferheads picking up a portion of an array of micro LED devices 300 inaccordance with an embodiment of the invention. In a particularembodiment illustrated in FIGS. 23-24, the pick up operation includespicking up the micro p-n diode 350, the metallization layer 320 and aportion of the conformal dielectric barrier layer 360 for the micro LEDdevice 300. In a particular embodiment illustrated in FIGS. 23-24, thepick up operation includes picking up a substantial portion of thebonding layer 310. Accordingly, any of the embodiments described withregard to FIGS. 19 and 22A-24 may also be accompanied by controlling thetemperature of the portion of the bonding layer 310 as described withregard to FIG. 18. For example, embodiments described with regard toFIGS. 19 and 22A-24 may include performing an operation to create aphase change from solid to liquid state in a plurality of locations ofthe bonding layer connecting the array of micro devices to the carriersubstrate 301 prior to picking up the array of micro devices. In anembodiment, the plurality of locations of the bonding layer can beregions of the same bonding layer. In an embodiment, the plurality oflocations of the bonding layer can be laterally separate locations ofthe bonding layer.

At operation 1950 the portion of the array of micro devices is thenreleased onto at least one receiving substrate. Thus, the array of microLEDs can all be released onto a single receiving substrate, orselectively released onto multiple substrates. For example, thereceiving substrate may be, but is not limited to, a display substrate,a lighting substrate, a substrate with functional devices such astransistors or ICs, or a substrate with metal redistribution lines.

FIG. 25 is a side view illustration of an array of micro device transferheads holding a corresponding array of micro LED devices 300 over areceiving substrate 401 including a plurality of driver contacts 410.The array of micro LED devices 300 may then be placed into contact withthe receiving substrate and then selectively released. FIG. 26 is a sideview illustration of the entire array of micro LED devices 300 releasedonto the receiving substrate 401 over a driver contact 410 in accordancewith an embodiment of the invention. In another embodiment, a subset ofthe array of micro LED devices 300 is selectively released.

In the particular embodiments illustrated in FIGS. 22A-26, the microdevices 300 are those illustrated in FIG. 27, example 270. However, themicro devices illustrated in FIGS. 22A-26 may be from any of the microLED device structures illustrated in FIGS. 27-29, and those described inrelated U.S. patent application Ser. No. 13/372,222.

FIG. 20 is a flow chart illustrating a method for picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention. At operation 2010 an array of compliant transfer heads ispositioned over an array of micro devices. The compliant transfer headsmay be any transfer head described herein. At operation 2020 the arrayof micro devices are contacted with the array of transfer heads.

At operation 2030 the sensor element is used to measure the degree ofdeflection of the spring portion of each transfer head. The sensor maymeasure that the deflection of the spring portion is within an expectedrange, or alternatively that the deflection exceeds or falls below theexpected amount. FIG. 30 illustrates a cross-sectional view of an arrayof transfer heads 200A-200D in contact over an array of micro devices300A, 300B, 300D. In an embodiment, transfer head 200A is in contactwith the surface of a micro device 300A, causing the spring portion todeflect an amount D into space 212. In another embodiment, transfer head200B is in contact with contamination particle 400 on the surface ofmicro devices 300B, causing the spring portion to deflect an amount D+Xinto space 212. In another embodiment, there is no micro device in thearray position corresponding to transfer head 200C, so that transferhead 200C has not deflected any amount into space 212. In yet anotherembodiment, the surface of micro device 300D is irregular or damaged,such that deflection of transfer head 200D is outside the expectedrange.

At operation 2040, a voltage is selectively applied to those transferheads that have deflected within the target range identified asindicating good contact for micro device pick up. In an embodiment, thepull in voltage is not applied to transfer heads that have deflected toa degree greater than the target amount of deflection, to transfer headthat have deflected less than the target amount of deflection, or toboth. In an embodiment, the pull in voltage is applied to all transferheads in the array. At operation 2050, a portion of micro devices ispicked up corresponding to the selectively activated portion of microdevice transfer heads. At operation 2060 the portion of the array ofmicro devices is then released onto at least one receiving substrate.

FIG. 21 is a flow chart illustrating a method of picking up andtransferring an array of micro devices from a carrier substrate to atleast one receiving substrate in accordance with an embodiment of theinvention. Each transfer head in the array has a base substrate, aspring member including a spring anchor coupled to the base substrateand a spring portion comprising an electrode where the spring portion isdeflectable into a space between the spring portion and the basesubstrate, and a dielectric layer covering the top surface of theelectrode as described in the above embodiments. At operation 2110 eachtransfer head in an array of micro device transfer heads is fullydepressed. An array of transfer heads may be depressed by, for example,positioning the transfer head array above a flat surface, contacting thearray with the flat surface with sufficient pressure to depress eachtransfer head until the backside electrode 134, 234 of each transferhead contacts the dielectric layer 136, 236 covering the opposingelectrode 138, 238 on the base substrate 102, 202.

At operation 2120, each transfer head is locked in the depressedposition by applying a voltage across each set of electrodes to lockeach transfer head in the depressed position. Where a flat surface hasbeen used to depress the array of transfer heads, the transfer head maythen be removed from the flat surface. At operation 2130, the lockingvoltage is removed from a portion of the transfer heads in order torelease them from the depressed position. The selectively releasedtransfer heads are then poised to pick up micro devices. FIG. 31illustrates a cross-sectional view of an array of micro device transferheads 200, where a portion of the transfer heads 200A, 200D are lockedin the depressed position, and a portion of the transfer heads 200B,200C have been selectively released from the depressed position.

At operation 2140, the selectively released array of transfer heads ispositioned over an array of micro devices. At operation 2150 the arrayof micro devices are contacted with the array of selectively-releasedtransfer heads. At this operation, only those transfer heads that havebeen selectively released from the depressed position contact thecorresponding micro device in the micro device array. Those transferheads that remain locked in the depressed position do not contact thesurface of a corresponding micro device. In an alternative embodiment,the selectively-released array of transfer heads is positioned over thearray of micro devices with a suitable air gap separating them whichdoes not significantly affect the grip pressure between theselectively-released transfer heads and the corresponding portion ofmicro devices. The arrays may be separated by an air gap distance of,for example, 1 nm (0.001 μm) or 10 nm (0.01 μm).

A voltage may then be applied to the array of transfer heads 200 atoperation 2160. In an embodiment, the pull in voltage is applied to alltransfer heads in the array. In another embodiment, the pull in voltageis applied only to transfer heads that have been selectively releasedfrom the depressed position. At operation 2170 a corresponding portionof the array of micro devices is picked up with the portion of the arrayof transfer heads that have been selectively released from the depressedposition. At operation 2180 the portion of the array of micro devices isthen released onto at least one receiving substrate.

In utilizing the various aspects of this invention, it would becomeapparent to one skilled in the art that combinations or variations ofthe above embodiments are possible for forming a micro device transferhead and head array, and for transferring a micro device and microdevice array. Although the present invention has been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the invention defined in the appended claims isnot necessarily limited to the specific features or acts described. Thespecific features and acts disclosed are instead to be understood asparticularly graceful implementations of the claimed invention usefulfor illustrating the present invention.

What is claimed is:
 1. A transfer head array comprising: a basesubstrate; an array of spring members, each spring member including: aspring anchor coupled to the base substrate; a spring portion includinga pair of electrically separate electrodes that are deflectable towardsthe base substrate; and a dielectric layer covering the pair ofelectrically separate electrodes.
 2. The transfer head array of claim 1,wherein each spring member comprises a mesa structure that protrudesaway from the base substrate.
 3. The transfer head array of claim 2,wherein each mesa structure includes a planar top surface.
 4. Thetransfer head array of claim 1, wherein each spring member comprises apair of mesa structures that protrudes away from the base substrate. 5.The transfer head array of claim 4, wherein each mesa structure includesa planar top surface.
 6. The transfer head array of claim 1, wherein thespring portion is deflectable into a space between the spring portionand the base substrate.
 7. The transfer head array of claim 6, whereinthe space is in the base substrate.
 8. The transfer head array of claim6, wherein the spring portion comprises a cantilever beam.
 9. Thetransfer head array of claim 6, wherein the spring portion comprisesmultiple cantilever beams.
 10. The transfer head array of claim 9,wherein each cantilever beam comprises a bend along a length of thecantilever beam.
 11. The transfer head array of claim 10, wherein eachcantilever beam comprises multiple bends along a length of thecantilever beam.
 12. The transfer head array of claim 6, wherein thespring portion comprises a membrane over the space.
 13. The transferhead array of claim 1, wherein the spring portion comprises silicon. 14.The transfer head array of claim 8, wherein the base substrate comprisessilicon.
 15. The transfer head array of claim 1, wherein the dielectriclayer comprises a material selected from the group consisting ofaluminum oxide and tantalum oxide.